Research Project:
CONTROL OF RUSTS OF CEREAL CROPSTitle: Virulence, frequency, and distribution of races of Puccinia striiformis f. sp. tritici and P. striiformis f. sp. hordei identified in the United States in 2008 and 2009

Interpretive Summary: Stripe rust of wheat is one of the most important disease in the U.S. and stripe rust of barley can damage crops in the western U.S. The pathogen populations can change race frequencies and distribution and develop new races. Monitoring race changes is important for breeding resistant cultivars and disease management. This study was conducted to charcaterize virulences, distributions, frequencies, and diversities of both wheat and barley stripe rust races in the U.S. in 2008 and 2009. In 2008, 33 races of wheat stripe rust were detected including a new race. In 2009, 26 races of wheat stripe rust were identified, including two new races. the distributions and frequencies of these races were determined for various epidemiological regions as well as the whole country. More races of wheat stripe rust were detected in the regions west than in the regions east of the Rocky Mountains, and similarly, the virulence diversity was higher in the west than in the east in the two years. For barley stripe rust, 11 races, including a new race, were detected in 2008 and 6 previously identified races were detected in 2009. The distributions and frequencies of the wheat and barley stripe rust races were discussed in comparison with the race data in the past. The information on changes of races and their distributions and frequencies should be useful in selecting genes for developing cultivars with effective stripe rust resistance and for the disease management.

Technical Abstract:
Puccinia striiformis f. sp. tritici (PST) and P. striiformis f. sp. hordei (PSH), the causal agents of stripe rust on wheat and barley, respectively, can change rapidly in virulence, which may overcome resistance in cultivars and result in severe epidemics. To monitor virulence changes in the pathogen populations, isolates obtained from stripe rust samples collected by the authors and collaborators from 17 states in 2008 and 13 states in 2009 in the U.S. were tested on 20 wheat and/or 12 barley differential genotypes to identify races of PST and PSH, respectively. In 2008, 33 PST races were detected including a new race, PST-138, which was similar to previous identified PST-127 (virulent on wheat differentials 1, 2, 3, 4, 6, 8, 9, 10, 11, 12, 13, 15, 16, 17, 18, 19, and 20) but not virulent on differential 8. The five most frequent races were PST-114 (virulent on differentials 1, 3, 4, 8, 9, 10, 11, 12, 14, 16, 17, 18, 19, and 20), PST-100 (virulent on differentials 1, 3, 8, 9, 10, 11, 12, 16, 17, 18, 19, and 20), PST-116 (similar to PST-114 plus virulent on differential 5), PST-101 (similar to PST-100 plus virulent on differential 2), and PST-98 (similar to PST-100 but not virulent on differential 9). In 2009, 26 PST races were identified, including two new races, PST-139 and PST-140. PST-139 was similar to PST-127 but not virulent on differentials 16 and 20. PST-140 was similar to PST-114 but not virulent on differential 9. The five most predominant races were PST-139 (19%), PST-140 (14%), PST-114 (11%), PST-116 (10%), and PST-127 (9%). However, the most widely distributed races were PST-98 (in 10 of the 14 states) and PST-102 (in 7 of the 14 states). Differential genotype AvSYr5NIL (Yr5) was the only one among the 20 differentials that was still resistant to all of the identified races. Virulence diversity of the PST populations was higher in the regions west than the regions east of the Rocky Mountains. For barley stripe rust, PSH-33 (virulent on barley differentials 1 and 7) was the most predominant (46%) among the 11 races detected in 2008, including a new race, PSH-82 (virulent on only barley differentials 1 and 11). In 2009, six previously identified races were detected, of which five (PSH-16, PSH-38, PSH-46, PSH-54, and PSH-71) were detected in Washington and two (PSH-54 and PSH-70) in Oregon. The information of the PST and PSH races should be useful in selecting genes for developing cultivars with effective stripe rust resistance.